One large tree crown can be defined as a local hotspot for plant species diversity in a forest ecosystem: a case study in temperate old-growth forest

Large, trees have survived for a long time, and their complex crown structure can serve as habitat for epiphytic plants. Canopy plants are not as well studied in the temperate zones as in the tropics, because many of them are accidental epiphytes, epiphytic individuals of normally terrestrial species. We hypothesized that the canopy can serve as a refuge for terrestrial species that have difficulty establishing on the ground (e.g., insufficient light, deer over-browsing), promoting and conserving forest species diversity. Terrestrial species may also vary in their ability to adapt to a wide range of canopy growth conditions. Here, we investigated canopy vascular plants hosted on a large Cercidiphyllum japonicum tree in a temperate old-growth forest, Japan. The canopy plant community was diverse comprising 39 vascular plant species (6 of which were threatened species), including 31 accidental epiphytes and 8 obligate epiphytes. We found at least 14 species could escape deer over-browsing by establishing in the canopy. Most accidental epiphytes were found on multiple and large reiterated trunks bifurcated from the main trunk as well as on large horizontal branches. Leaves of canopy plants had higher nitrogen concentration compared to those of plants on the ground, and higher water-use efficiency with increasing height was achieved by controlling transpirational water loss. Our results show one large tree crown can be defined as a local hotspot for current and future plant species diversity in a temperate old-growth forest, reinforcing its ecological value for conservation purposes.